* Astronomy

On February 24, 2007, the LEISA infrared spectral imager in the New Horizons Ralph instrument observed giant Jupiter in 250 narrow spectral channels. At the time the spacecraft was 6 million kilometres from Jupiter; at that range, the LEISA imager can resolve structures about 400 kilometres across.
That may seem large, mission scientists say, but Jupiter itself is more than 144,000 kilometres across.


Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

"The detail revealed in these images is simply stunning. Our instrument is performing spectacularly well" - Dr. Dennis Reuter, Ralph/LEISA project scientist and a New Horizons co-investigator from NASA's Goddard Space Flight Center in Greenbelt, Md.

LEISA observes in 250 infrared wavelengths, which range from 1.25 micrometers (µm) to 2.50 µm. The three images shown above from that dataset are at wavelengths of 1.27 µm (left), 1.53 µm (centre) and 1.88 µm (right).

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On February 28, 2007, NASA's New Horizons spacecraft made its closest approach to Jupiter on its ultimate journey to Pluto. This flyby gave scientists a unique opportunity to study Jupiter using the package of instruments available on New Horizons, while coordinating observations from both space- and ground-based telescopes including NASA's Chandra X-ray Observatory.
In preparation for New Horizon's approach of Jupiter, Chandra took 5-hour exposures of Jupiter on February 8, 10, and 24th. In this new composite image, data from those separate Chandra's observations were combined, and then superimposed on the latest image of Jupiter from the Hubble Space Telescope.


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Credit: X-ray: NASA/CXC/SwRI/R.Gladstone et al.; Optical: NASA/ESA/Hubble Heritage (AURA/STScI)

The purpose of the Chandra observations is to study the powerful X-ray auroras observed near the poles of Jupiter. These are thought to be caused by the interaction of sulphur and oxygen ions in the outer regions of the Jovian magnetic field with particles flowing away from the Sun in the so-called solar wind. Scientists would like to better understand the details of this process, which produces auroras up to a thousand times more powerful than similar auroras seen on Earth.
Following closest approach on the 28th, Chandra will continue to observe Jupiter over the next few weeks. New Horizons will take an unusual trajectory past Jupiter that takes it directly down the so-called magnetic tail of the planet, a region where no spacecraft has gone before. The sulphur and oxygen particles that dominate Jupiter's magnetosphere and originate in Io's volcanoes are eventually lost down this magnetic tail. One goal of the Chandra observations is to see if any of the X-ray auroral emissions are related to this process.
By combining Chandra observations with the New Horizons data, plus ultraviolet information from NASA's Hubble Space Telescope and FUSE satellite, and optical data from ground-based telescopes, astronomers hope to get a more complete picture of Jupiter's complicated system of particles and magnetic fields and energetic particles. In the weeks and months to come, astronomers will undertake detailed analysis of this bounty of data.

Title: A study of Jupiter's aurorae with XMM-Newton
Authors: G. Branduardi-Raymont (1), A. Bhardwaj (2), R. F. Elsner (3), G. R. Gladstone (4), G. Ramsay (1), P. Rodriguez (5), R. Soria (6 and 1), J. H. Waite Jr (7), T. E. Cravens (8) ((1) Mullard Space Science Laboratory, University College London, Holmbury St Mary, UK, (2) Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, India, (3) NASA Marshall Space Flight Center, Huntsville, USA, (4) Southwest Research Institute, San Antonio, USA, (5) XMM-Newton SOC, Villafranca, Spain, (6) Harvard-Smithsonian Center for Astrophysics, Cambridge, USA, (7) University of Michigan, Ann Arbor, USA, (8) Department of Physics and Astronomy, Lawrence, USA)

We present a detailed analysis of Jupiter's X-ray (0.2-10 keV) auroral emissions as observed by XMM-Newton in Nov. 2003 and compare it with that of an Apr. 2003 observation. We discover the existence of an electron bremsstrahlung component in the aurorae, which accounts for essentially all the X-ray flux above 2 keV: its presence had been predicted but never detected for lack of sensitivity of previous X-ray missions. This bremsstrahlung component varied significantly in strength and spectral shape over the 3.5 days covered by the Nov. 2003 observation, displaying substantial hardening of the spectrum with increasing flux. This variability may be linked to the strong solar activity taking place at the time, and may be induced by changes in the acceleration mechanisms inside Jupiter's magnetosphere. As in Apr. 2003, the auroral spectra below 2 keV are best fitted by a superposition of line emission most likely originating from ion charge exchange, with OVII playing the dominant role. We still cannot resolve conclusively the ion species responsible for the lowest energy lines (around 0.3 keV), so the question of the origin of the ions (magnetospheric or solar wind) is still open. It is conceivable that both scenarios play a role in what is certainly a very complex planetary structure. High resolution spectra of the whole planet obtained with the XMM-Newton RGS in the range 0.5-1 keV clearly separate emission lines (mostly of Fe) originating at low latitudes on Jupiter from the auroral lines due to O. These are shown to possess very broad wings which imply velocities of ~5000 km/s. Such speeds are consistent with the energies at which precipitating and charge exchanging O ions are expected to be accelerated in Jupiter's magnetosphere. Overall we find good agreement between our measurements and the predictions of recent models.

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Astronomers from the University of California, Berkeley, and the W. M. Keck Observatory in Hawaii this month snapped high-resolution near-infrared images of the Great Red Spot, a persistent, high-pressure storm on Jupiter, as a smaller storm, Red Spot Jr., breezed by it on its race around the planet.

The image, which also shows Jupiter’s moon Io, was taken on July 20 Hawaii time (July 21 Universal Time) by the Keck II telescope on Mauna Kea using adaptive optics (AO) to sharpen the image.
The spots are of interest to astronomers because Red Spot Jr. formed from the merger of three white spots only recently, between 1998 and 2000, and in December 2005 turned red like the much older Great Red Spot. While the new red spot is about the size of Earth, the Great Red Spot is nearly twice that diameter and has been circling the planet for at least 342 years.
The images captured by the second-generation Near Infrared Camera (NIRC2) on Keck II show that, though the two red spots are about the same colour when seen in visible wavelengths (see Christopher Go’s optical image from July 20 UT), they differ markedly at infrared wavelengths. When the astronomers viewed the planet through a narrow-band filter centred on the 1.58 micron, near-infrared wavelength, Red Spot Jr., which was called Oval BA before it changed from white to red, was a lot darker, indicating that the tops of the storm clouds may be lower than those of the Great Red Spot. With more atmosphere above its cloud tops, more infrared light is absorbed by molecules like methane in the atmosphere.

"Red Spot Jr. is either not as high as the Great Red Spot, or it’s just not as reflective, that is, as dense. These images will put some constraints on the altitude of Red Spot Jr." - astronomer Imke de Pater, professor of astronomy at UC Berkeley.

The Great Red Spot is thought to tower about 8 kilometres above the surrounding cloud deck. The fact that Red Spot Jr. turned red may indicate its swirling storm clouds are rising higher also, though apparently they are not as high as those of its larger companion, or the clouds are thinner.

Why the spots are red is a subject of great debate. Some people think the hurricane-like winds in the Great Red Spot, which can reach 400 miles per hour, dredge up material from deeper in the planet’s atmosphere that, when exposed to ultraviolet light, turns red. One candidate is phosphine gas, PH3, which has been detected on Jupiter. Ultraviolet light might catalyse its conversion to red phosphorus, P4, according to one of the leading theories. Other, more complicated theories have phosphine interacting in the atmosphere with chemicals such as methane or ammonia to form complex compounds such as methylphosphane or phosphaethyne.
Recent studies suggest that the red colour also may be attributed to sulphur allotropes, that is, different molecular configurations, including chains and rings, of pure sulphur, such as S3-S20. The new work hypothesizes that ammonium hydrosulphide particles are carried upwards in the Great Red Spot and are broken up by ultraviolet light. Subsequent chemical reactions ultimately lead to long-chained sulphur allotropes , which can vary in colour from red to yellow.

"The jury is still out on the exact processes that lead to the red coloration of the Great Red Spot – and Oval BA" - Imke de Pater.

Christopher Go, an amateur astronomer who first noticed the coloration change of Red Spot Jr., joined de Pater’s team earlier this year. He noted that during the close encounter between the two spots, Red Spot Jr. was squashed slightly, stretching in its direction of motion. The same thing happened in 2002 and 2004 when the Great Red Spot and Red Spot Jr. passed one another, though then Junior was white.
The Great Red Spot rotates westward, opposite to the eastward rotation of the planet. Because alternating bands on the Jovian surface move in opposite directions, the adjacent Red Spot Jr. moves eastward. The planet rotates about once every 10 hours.
Another of de Pater’s colleagues, UC Berkeley mechanical engineering professor Philip Marcus, predicted several years ago that Jupiter’s climate was changing, based on the disappearance of the cyclonic storms or spots within the bands. The mixing of the atmosphere by these cyclones keeps the temperature about the same over the entire planet, he argued, so loss of this mixing will cause the equator to heat up and the poles to cool.
Earlier this year, on April 16, de Pater and her team captured near-infrared, ultraviolet and visible light photos of the planet using the Hubble Space Telescope to look more closely at the two red spots. The observations with the Keck Telescope were a follow-up study to try to measure the speeds of the swirling winds in the spots. Jupiter’s brightness, however, confused the adaptive optics system, forcing the astronomers to miss some good shots of the planet as the guide star was being positioned optimally relative to Jupiter.



"This was probably the most challenging observation ever tried with the AO system at Keck" - Imke de Pater, referring to use of the laser guide star system next to an object as bright as Jupiter.

Adaptive optics can take the twinkle out of an object caused by thermal motion in the atmosphere, but to do this well, the target must be near another bright object that can serve as a reference. For some of the images, Jupiter’s moon Io was used as the reference “star.” But until Io got close enough for this, a laser guide star was created near Jupiter to serve this purpose.

"This was our first attempt using the laser to obtain AO-corrected images of Jupiter’s surface. The technique shows promise and, if we perfect it, will provide us with many more opportunities to observe this fascinating, ever-changing object" - Dr. Al Conrad, a support astronomer at the Keck Observatory.

The team also obtained a close-up of the two spots through a narrow-band filter centred on 5 microns, which samples thermal radiation from deep in the cloud layer. Both spots appear dark because the clouds completely block heat emanating from lower elevations, though narrow regions around the spots that are devoid of clouds show leakage of this heat out into space.

"These 5 micron images reveal details in the cloud opacity not seen at the other wavelengths and will help unravel the vertical structure of the spots. The smooth, narrow arcs visible to the south of each spot probably result from the interaction between the spots and high-speed winds that are deflected around them" - Michael Wong, UC Berkeley team member.

The resolution using both the narrow and wide views on the camera was about 0.1 arcseconds, or only half as good as can be obtained on a clear night with optimal seeing.

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